Octets

Octets are numbers containing a binary sequence that indicates either the number or a continuation of said sequence.

Each octet is an 8 bit string, bits 1-8 can be either 1 or 0, bit 8 indicates whether it's a continuation (1) or a terminus (0).

Octets must be 0-128 or 255, examples are provided below.

1. The number 0. 00000000
2. The number 127. 11111110
3. The number 128. 10000000
4. The number 255. 11111111

You will have to create a function to parse numbers into a series of octets. As mentioned before, octets must have a length number containing the number of octets preceding the content of octets.

An example function is provided below.

$length = 0;
while(true){
if(!isset($number)){
$number = $element;
if($number > 128){
if(is_string($length)){
$length .= hex2bin(dechex(255));
} else {
$current_octet .= hex2bin(dechex(255));
}
$number = $number - 128;
} else {
$number = dechex($number);
if(strlen($number) & 1){
$number = "0".$number;
}
if(is_string($length)){
$length .= hex2bin($number);
$current_octet = $length.$current_octet;
$length = null;
$octets .= $current_octet;
$current_octet = "";
unset($number);
break;
} else {
$current_octet .= hex2bin($number);
}
}
if(!is_string($length)){
$length++;
}
if(is_string($number)){
if(isset($key) && isset($element[$key + 1])){
$key++;
$number = $element[$key];
continue;
} else if($length > 0){
$number = $length;
$length = "";
continue;
}
}
}
}

An example function to parse octets is provided below.

$data = "";
$current_position = 0;
while(true){
$number = hexdec(bin2hex(substr($data, $current_position, 1)));
$current_position++;
if($number === 255){
$element = $element + ($number >> 1) + 1;
} else if($number <= 128){
$element = $element + $number;
} else if ($number > 128){
//return exception;
}
}

The sequence of octets and what format to parse them in follows below.

1. Cipher: An array of octets parsed as numbers.
2. Original length: The result of all octets in the sequence added up.
3. Byte range: The result of all octets in the sequence added up.
4. Content Type: A string filled with all the octets in the sequence converted from ISO-10646-1 to UTF-8.
5. Original MD5 Checksum: A string filled with all the octets in the sequence converted from integer to a 2 byte hexadecimal string.
6. Size Array: An array of octets parsed as numbers, each number represents the length in bytes of the length character.

It is ultimately up to you on how to implement the octet creating/parsing function.